FIG. 8 is a circuit diagram illustrating a conventional power device 200. In the power device 200, an output transistor 204 is controlled such that a feedback voltage Vfb (a divided voltage of an output voltage Vout) and a predetermined reference voltage Vref are equivalent, whereby the desired output voltage Vout is generated from a power source voltage VCC and supplied to a load.
However, the power device 200 involves various problems to be solved, such as a trade-off between a restraint of a leak current of the output transistor 204 and low current consumption, a trade-off between a reduction in an internal power source voltage generation block (PREREG) 201 and low current consumption, etc.
<Trade-Off Between a Restraint of a Leak Current of Output Transistor 204 and Low Current Consumption>
Recently, in the power device 200, the size of the output transistor 204 in both a low drop-out (LDO) regulator IC and a switching regulator IC tends to be increased. If the size of the output transistor 204 is increased, it is likely that a leak current Ileak generated from the output transistor 204 is increased.
If a load is not connected to the power device 200, the leak current Ileak of the output transistor 204 flows through a single path, along which the leak current Ileak of the output transistor 204 flows to a ground terminal through feedback resistors 205 and 206 interposed between the output transistor 204 and the ground terminal. In many cases, a feedback resistance value Rfb (a combined resistance value of the feedback resistors 205 and 206) is set to be somewhat large in order to realize low current consumption of the power device 200. For this reason, if the leak current Ileak of the output transistor 204 flows to the feedback resistors 205 and 206, it is likely that the output voltage Vout is increased to be higher than an intended target value. For example, if the leak current Ileak is 1 μA and the feedback resistance value Rfb is 5 MΩ, the output voltage Vout is increased by 5V as a product of the leak current Ileak and the feedback resistance value Rfb.
In particular, the leak current Ileak of the output transistor 204 is increased as chip temperature Tj is increased. For this reason, the foregoing problem may occur at the surface in the power device 200 (e.g., a power source IC mounted in a vehicle), whose temperature may be high when used.
Further, the foregoing problem may be solved by setting the feedback resistance value Rfb to be small. However, if the feedback resistance value Rfb is set to be small, low current consumption of the power device 200 cannot be realized. Thus, it is not practical to set the feedback resistance value to be small. In addition, the size of the output transistor 204 may be reduced or the power device 200 may be restrained from having a high temperature to suppress the leak current Ileak of the output transistor 204. However, the use of the above mentioned methods brings about another trade-off (i.e., it causes an increase of ON resistance of the output transistor 204, etc).
<Trade-Off Between Reduction in Size of Internal Power Source Voltage Generation Block 201 and Low Current Consumption>
FIG. 9 is a circuit diagram illustrating a conventional reference current generation circuit 300 included in the internal power source voltage generation block 201. In order to reduce current consumption as much as possible in generating the reference current Iref, the reference current generation circuit 300 is configured such that a resistance value of a resistor Rx is set to be great to thus reduce a bias current Ix (a drain current of a transistor M10) flowing at an input side of a current mirror. Thus, in the reference current generation circuit 300, an increase in the resistance value of the resistor Rx leads to an increase in the area of the chip. For example, in order to narrow down the bias current Ix flowing through the resistor Rx to 0.1 μA, a resistance value of the resistor Rx should be set to be tens to hundreds of MΩ (which is equivalent to 10 or more aluminum pads), and this hampers the reduction of the size of the internal power source voltage generation block 201.
Further, in the reference current generation circuit 300, as the power source voltage VCC becomes higher, the bias current Ix is increased. Thus, in order to limit the current consumption of the reference current generation circuit 300 to be small while responding to the input of the high power source voltage VCC, the resistance value of the resistor Rx is required to set to be larger than the above value and the chip area is required to be increased further.